1. Field of the Invention
The present invention relates to the field of optical and magneto-optical information storage systems and, more particularly, to a composite optical assembly for applying a light spot to information storage media and retrieving information from the media optically.
2. Description of the Prior Art
Information storage systems, particularly those used with computer systems, typically store data magnetically or optically on to several types of storage media, rotating magnetic or optical disks for example. Such storage media may include information for document files, computer output memories and the like which are used for recording and retrieval only, or media which permits recording, retrieval or erasure of information. The data stored on such media, whether magnetic or optical is contained within a series of tracks. Once formatted on a disk, such tracks are spiral or concentric about the disk center and may number in the thousands of tracks per disk side. The total number of tracks enhance the storage capacity of the disk and depends upon the diameter of the disk utilized and the method of recordization, either magnetically or optically, of the data.
In both magnetic recording and magneto-optical recording, information is stored on a storage disk by orientating the magnetization of the media at given points along the tracks. In order to access these tracks in order to record and/or read data on a disk, a read/write head or transducer for magnetic recording or an optical assembly for optical recording, comprising at least a spot forming mechanism in the case of optical recording, is moved along a generally radial path across the surface of the storage disk a the storage disk is being rotated. The generally radial movement of the transducer or optical assembly will follow a straight line path or an arcuate path, depending upon whether a linear or rotary actuator is utilized to position the head.
In magnetic-optical recording, information is encoded and is stored in a sequence of magnetic domains oriented normal to the storage media surface in either of two possible orientations, north pole up or north pole down for example.
In an optical and/or magneto-optical media reading apparatus, an optical assembly is employed which applies the light spot to the data storage media. In this optical assembly a laser beam typically generated by semiconductor laser or other suitable light source can be focused on this storage medium by an objective lens and is reflected by the media. The laser beam when reflected from the media passes through the objective lens and is then detected by a photo detector or other suitable detector. The detected signal is then processed to extract the information as reproduced by the magneto-optical effect, particularly the Kerr effect, the phenomenon that the plane of polarization of a linearly polarized light beam is rotated when the light beam is reflected from the surface of a suitable magnetic recording medium.
The basic construction of a magneto-optical disk apparatus according to the prior art is shown in the magneto-optical information reading apparatus depicted in FIG. 1 which comprises a means for causing a light beam (I.sub.0) to be polarized in a predetermined direction by passing the beam through a polarizing plate 10, through a half mirror 20, onto a magnetic recording medium 30, reflecting the beam therefrom back to the half mirror 20 where a portion of the beam is directed out through an analyzer 40 into a photodectector 50.
Another example of the prior art but incorporating differential detection of the reflected signal is shown in FIG. 2 of the accompanying drawings. In FIG. 2 a light beam emitted from a semiconductor laser 60 is linearly polarized in the direction of peak polarization and is then collimated by collimating lens 62 and then passes through beam splitter 64 (either the half mirror amplitude type or the so called leaky beam splitter type). The light beam is imaged as a minute spot on the magneto-optical recording medium 66, by objective lens 65. When a magnetic domain is formed on the media 66, as shown in FIG. 2, the light beam reflected from the medium 66 is subjected to a rotation in the plane of polarization of .+-..THETA..sub.k caused by the Kerr effect in conformity with whether the direction of magnetization of the domain is upwards or downwards.
The magneto-optically modulated reflected light is again collimated by object lens 65 and reflected by half mirror 64, whereafter it is optically formed into a convergent light beam by the condensing lens 68 which is divided by half mirror amplitude beam splitter 70 with the divided light then passing through analyzers 72, 74 respectively to be detected as intensity modulated light beams 76, 78 by photo detectors 80, 82.